1. Field of the Invention
The present invention relates to a frequency modulation circuit. More specifically, the present invention relates to a stabilizing circuit for stabilizing a center frequency of a VCO (voltage-controlled oscillator) circuit which is included in a PLL (Phase-Locked Loop) constituting a frequency modulation circuit for frequency-modulating a sound signal, for example.
2. Description of the Prior Arts
For example, in Japanese Patent Laying-open No. 59-3709 (GllB 5/04), as shown in FIG. 3, there is disclosed a configuration wherein an output of a frequency modulation circuit 1 which is constructed by a VCO is frequency-divided to a very low frequency by a frequency divider 2 to be phase-compared with a reference signal of 30 Hz in a phase comparator 3, being added to an original sound signal and supplied to the frequency modulation circuit 1. The reference numeral 4 denotes a capacitor of a loop filter which constitutes a PLL along with the frequency modulation circuit 1. A reason why the phase-comparison is performed in a low frequency region is that it is necessary to prevent the control of the center frequency of the frequency modulation circuit from being influenced by a sound signal input. Therefore, it is necessary to sufficiently lower a cut-off frequency of the loop filter.
A circuit configuration of a specific loop filter which can be utilized in the PLL circuit is shown in FIG. 4. A loop filter shown in FIG. 4 is mainly constructed by an operational amplifier 9, and a front stage 10 determines the step-responsiveness of the PLL circuit and a rear stage 11 determines the stability of the filter in a steady state. Using symbols in FIG. 4, a characteristic of the loop filter can be represented as follows: ##EQU1##
However, if such a circuit configuration is adopted in an integrated circuit, since a capacitance of the capacitor is large in a case of a frequency modulation circuit which processes a sound signal, it is necessary to connect the capacitor externally, and therefore, there is a disadvantage that the number of necessary pins (terminals) of the integrated circuit becomes large.
A circuit configuration of FIG. 5 is for solving the above described disadvantage, and the same can reduce the number of necessary pins and perform the same or similar characteristic as that of FIG. 4. A characteristic of the circuit is as follows: ##EQU2## where C.sub.1 =nC.sub.2.
In this case, the freedom of a circuit design, for example, the relationship between C.sub.1 and C.sub.2 is limited; however, the same characteristic as that of FIG. 4 can be obtained.
However, in practice, a characteristic in a high-frequency region of the operational amplifier 9 is different from an ideal state thereof, and therefore, in FIG. 5 circuit configuration, the operational amplifier becomes to function as a merely bypass capacitor with respect to high-frequency components, for example, an edge portion of a phase-comparing signal.
On the other hand, as a kind of a loop filter capable of implementing a similar characteristic, a lowpass filter which is operated by a current source is known. One example is shown in FIG. 6. Using symbols in FIG. 6, characteristic can be represented as follows: ##EQU3##
The circuit of FIG. 6 is similar to the circuit of FIG. 4. Practically, the operational amplifier 9 may be not used and a circuit configuration of FIG. 7 is enough to operate as a loop filter. In a case of FIG. 7 circuit configuration, a characteristic can be represented as follows: ##EQU4## where C.sub.1 =nC.sub.2.
If FIG. 7 circuit configuration is utilized, the number of necessary pins may be small and the above described disadvantage of FIG. 5 can be solved.
A problem in the loop filter which is operated by a current source and shown in FIG. 7 is that since a capacitor C.sub.1 is connected to the ground, in a case where a power source is a single power source of a positive polarity, the capacitor C.sub.1 is charged through a resistor R.sub.2, and it takes a lot of time until a terminal voltage Vb of the capacitor C.sub.1 becomes approximately the same as a voltage Va. Therefore, in turning a power source on, a pull-in of the center frequency is delayed.